Non-custom ceramic dental products such as artificial teeth are readily available from commercial manufacturing sources for use in the fabrication of intra-oral restorative devices. However, there are a number of types of ceramic dental products which require individual custom-type production to satisfy certain requirements for an individual patient. One such product includes ceramic jacket crowns which generally are fabricated by a multi-layered build-up technique on a metal substrate, the individual layers of which must be dried and successively fired before the application of the next outer layer.
Further types of ceramic dental products including porcelain facings, veneers, porcelain bridges, porcelain inlays, and various other porcelain dental products are also generally fabricated by a successive step technique in which a layer is applied to a base member of suitable type and fired. Thereupon, additional layers are applied and fired. In this manner a finished product of required shape, shade or color, and dimension for particular use in a restorative device may be fabricated.
Each of the porcelain layers usually are applied in the form of a slurry, which is somewhat paste-like. The slurry is then dried, prior to being fused or fired, to remove moisture from the condensed porcelain thereby substantially to obviate the possibility that the porcelain will explode from the metal substrate during fusing or firing. Preferably, heat is used to finish the drying of the applied slurry layer as rapidly as possible so as to conserve the amount of time required to complete the ceramic dental product.
The drying of such applied layers may be carried out in the same or different furnace from that in which firing or vitrification of the various layers occurs. In the former case the final firing temperature usually is not applied until satisfactory evacuation of the product has occurred. Satisfactory evacuation of the product has in the past been the result of "guesstimation," albeit educated "guesstimation." To this end, the coated metal substrate or crown generally is placed within and spaced from the open door of the furnace by a distance of a couple of inches. Over a period of time the crown is moved closer to the door until it is considered that the crown has been sufficiently dried. At this time the crown is repositioned to the rear of the furnace and subjected to a greater temperature to accomplish a fusing or firing of the crown.
This procedure, while it may be carried out successfully by trained personnel, leaves something to be desired from the standpoint of the overall manufacturing procedure. Even if trained personnel are utilized at this stage of the operation, distraction because of other duties may result in inconsistency of product, a result of variation in time during which the drying operation is carried out. As already discussed, a crown which is improperly dried, i.e., one not dried sufficiently, may explode from or crack on the metal substrate when subjected to firing temperature. On the other hand, if the crown is dried for too long a period it may begin to bisque bake and upon full firing attain an incorrect opacity (transparency) or an undesirable color. If during the initial drying period which may continue for a period of time longer than desired the drying temperature is at or near to a temperature within the firing range the porcelain may flake off upon movement into the firing furnace.
The above procedure of drying a crown is not compatible with production efficiency. If the operator's time is consumed by the necessity both of checking product and determining through past experience whether suitable drying of the product has been carried out that operator may not be able adequately to carry out other tasks which will serve as a distraction from the principal endeavor.